The present invention relates to the fields of threaded fasteners and of methods of fabrication thereof, and to installing mine roof support systems using nuts having a portion which is deformable upon application of excess torque.
U.S. Pat. No. 4,662,795 of Clark et al, issued May 5, 1987 and assigned to applicant's assignee, describes a method of supporting and reinforcing a rock structure such as a mine roof utilizing a nut having an internally threaded portion and an integral, domed portion. Fabrication of the nut includes forming a metal casting having through a bore with first and second contiguous portions over its axial length. The first portion is drilled and tapped to provide a threaded portion having a first diameter, less than the diameter of the substantially cylindrical surface of the second portion of the bore. At least a portion of the external surface of the nut outwardly adjacent the threaded portion of the bore is formed with a plurality of flat sides for engagement by a wrench. The external surface outwardly adjacent the second portion of the bore is formed in the casting as substantially cylindrical and is deformed by an appropriate die to a domed configuration having an opening in the top of the dome of smaller diameter than the first diameter.
A first, threaded end of a bolt is inserted through an opening in a conventional bearing plate and threaded into the nut until the end of the bolt contacts the inside of the domed portion about the periphery of the opening. A standard resin cartridge is placed in a bore hole of predetermined depth and diameter in the rock formation, followed by the end of the bolt opposite that engaged with the nut. The cartridge is crushed against the end of the bore hole by advance of the bolt and its contents released.
Rotation of the bolt by a wrench engaged with the nut serves to mix the resin components. Rotation is stopped for a time, usually not more than 10 seconds, to allow the mixed components to harden and thus anchor the bolt within the drill hole. Application of excess torque to the nut, with rotation of the bolt prevented due to its engagement by the hardened resin, causes the end of the bolt to break through the domed portion of the nut. Sufficient torque is applied to thread the nut into secure engagement with the bearing plate and tension the bolt to the desired degree.
Although the above-described method of mine roof support has received large and widespread acceptance throughout the industry, it has been found that the torque required to break the end of the bolt through the domed end of the nut is sometimes excessive. Efforts to alleviate this problem by increasing the size of the opening or decreasing the thickness of the material at the domed end of the nut have not proven effective. When the torque required to break the end of the bolt through the domed end of the nut is not consistently at or near the desired level, reliability of operation and integrity of the anchorage are threatened.
Accordingly, it is a principal object of the present invention to provide a novel and improved threaded fastener in the nature of a domed nut, and a method of fabrication thereof, for use with conventional elongated bolts, bearing plates and resin cartridges in mine roof support systems.
Another object is to provide a method of supporting and reinforcing a mine roof utilizing a threaded bolt, a bearing plate, a resin cartridge and a domed nut wherein the torque required to cause the end of the bolt to break through the domed end of the nut is consistent, at a level less than that required for prior art systems of the same general type.
Other objects will in part be obvious and will in part appear hereinafter.